How to define struct member as a sum of other members - c

Question

There are some questions similar to this (namely, Define a struct with a member pointing to anothermember), but, given I'm complete noob at C, I cannot satisfy my doubt reading them, so I'm opening my own in the expectation of having some difference on it (if not, please point me to a proper question and close this as dupe).
What I'm trying to achive is self explanatory:
typedef struct {
int goals_as_visitor;
int goals_as_home;
int total_goals = goals_as_visitor + goals_as_home;
} team
I just want to store the total_goals struct member as permanently being the sum of the other two members. The above code doesn't compile. I don't know if I just haven't found how to do it just yet, or if this is not possible.
Any guidance?

Answer 1

No this is not possible. A C definition is roughly a definition of a storage space, no more. If you want to ensure that the field is always the sum of the first ones, then you need a discipline: each time you modify one of them, you'll have to modify the third. In this regard, OOP languages are better at this, C is too basic. Anyway you can try to use OOP style in C, something like:
typedef struct Team {
int goals_as_visitor;
int goals_as_home;
int total_goals;
} team;
void setGoalAsVisitor(struct Team *this,int value) {
this->goals_as_visitor = value;
this->total_goals = this->goals_as_visitor + this->goals_as_home;
}
...
setGoalAsVisitor(&team,666); // Roughly calling a method on team: aka team.setGoalAsVisitor(666) in Java style
...
and discipline yourself not to use the fields directly.
Note: you can hide many thing to enforce more the discipline, but alas C can let you make nasty things and violate the rules...

Answer 2

Here you are just defining a structures by telling the compiler what members it contains.
To use this structure first you have to declare the structure and then and then only will a memory be allocated for that structure.
Structure members cannot be initialised with declaration.
Definition would be like this:
typedef struct {
int goals_as_visitor;
int goals_as_home;
int total_goals;
} team
and declaration will be like:
team red;
team blue;
Now the structure has some memory allocated, and structure members can be accessed using dot [.] operator.
e.g. red.goals_as_visitor = 10;
Here is a sample code may be this can help you out
#include <stdio.h>
typedef struct {
int goals_as_visitor;
int goals_as_home;
int total_goals;
}team;
int main (void)
{
team red ;
red.goals_as_visitor=10;
red.goals_as_home = 5;
red.total_goals = red.goals_as_visitor + red.goals_as_home;
printf("%d %d %d\n", red.goals_as_visitor, red.goals_as_home, red.total_goals);
return 0;
}
You can refer below link for basics of structures [https://www.geeksforgeeks.org/structures-c/]

Answer 3

It is not possible in c.But the total_goals will always sum of goals_as_visitor and goals_as_home.So the better way is to use total_goals as a function pointer like this
#include<stdio.h>
/*structure and elements*/
struct team;
int tot_g(struct team *sthis);
typedef int (*tot_gol)(struct team *sthis);
struct team
{
int goals_as_visitor;
int goals_as_home;
const tot_gol total_goals;
}
const init={.goals_as_visitor=0,.goals_as_home=0,.total_goals=tot_g};
int tot_g(struct team *sthis)
{
return (sthis->goals_as_visitor+sthis->goals_as_home);
}
typedef struct team team;
/*structure and elements*/
int main()
{
team p=init;
p.goals_as_home=5;
p.goals_as_visitor=5;
printf("%d",p.total_goals(&p));
}
You must always initialize when a team object is defined with init.
Or
The simple way to do this is using a function like
#include<stdio.h>
typedef struct {
int goals_as_visitor;
int goals_as_home;
} team;
int total_goals(team var)
{
return (var.goals_as_home+var.goals_as_visitor);
}
int main()
{
team p;
p.goals_as_home=5;
p.goals_as_visitor=5;
printf("%d",total_goals(p));
}
You can use total_goals function parameter as const pointer also.